In the area of digital printing (the term "printing" is used to encompass both printing and displaying throughout), gray level has been achieved in a number of different manners. The representation of the intensity, i.e., the gray level, of a color by binary displays and printers has been the object of a variety of algorithms. Binary displays and printers are capable of making a mark, usually in the form of a dot, of a given, uniform size and at a specified resolution in marks per unit length, typically dots per inch. It has been common to place the marks according to a variety of geometrical patterns such that a group of marks when seen by the eye gives a rendition of an intermediate color tone between the color of the background (usually white paper stock) and total coverage, or solid density.
Continuous tone images contain an apparent continuum of gray levels. As an approximation to continuous tone images, pictorial imagery has been represented via binary halftone technologies. In order to record or display a halftone image with a scanning system, one picture element of the recording or display surface consists of a j.times.k matrix of sub-elements where j and k are positive integers. A halftone image is reproduced by printing the respective sub-elements or leaving them blank, in other words, by suitably distributing the printed marks.
Halftone image processing algorithms are evaluated in part, by their capability of delivering a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency rendition (fine detail) with high contrast modulation makes that procedure superior to one which reproduces such fine detail with lesser or no output contrast.
Another method of producing gray levels is provided by gray level printing. In such a method, each pixel has the capability to render several different dot sizes. The dot size for a pixel is a function of the exposure time provided an LED element corresponding to that pixel. The longer the exposure time, the more toner is attracted to that particular pixel. See, for example, U.S. Pat. No. 4,680,645 for a method of rendering gray scale images with variable dot sizes.
There are two major concerns in rendering a continuous tone image for printing: (1) the resolution of image details, and (2) the reproduction of gray scales. In a binary halftone representation scheme, these two fundamental factors compete with each other. The more gray levels that are rendered, the larger is the halftone cell. Consequently, coarse halftone line screens are provided, with the attendant poor image appearance. Hence, a compromise is made in rendering between the selection of line resolution and gray scales in binary halftone printing. However, in gray level halftone printing, one can satisfy both resolution and gray level requirements. In gray level printing, the same number of addressable dots are present, and there is a choice of dot sizes from one dot-size of 1 bit/pixel to 16 different dot-sizes of 4 bit/pixel. An image could then be rendered with 133 line screens and 128 gray scales of higher quality image. Although providing higher image quality with respect to line resolution and tonal scales, gray level halftoning presents its own dot rendering issues.
A problem exists in the application of a gray level rendering technique to a document that contains different types of images: text, halftone, and continuous tone. These different types of images create different rendering problems, based on a trade-off between tone scales and detail resolution. For example, with text, the number of tone scales is not as important as providing a smooth text edge, whereas the opposite holds true for continuous tone images. Providing a single type of gray level halftone rendering technique to a document that contains two or more types of images may lead to the production of a document in which one or more of the different types of images are reproduced unsatisfactorily.
As indicated earlier, continuous-tone pictures can only be printed in binary form through a halftoning process. The halftone process breaks the picture into dots via a screen-like structure. Through the integration of the human visual system, a sensation of gray shades is achieved. Normally, increasing the dot resolutions (for example, 2000 dots per inch (dpi)-3000 dpi or higher) and making the dots smaller are the way to produce a high quality picture (a continuous-tone like picture). However, it is not necessary to present the continuous tone photographic quality picture in such high image resolutions. For example, 400 dpi or 500 dpi printing resolutions with 8-bit to 12-bit gray scales are adequate for true continuous tone photographic quality printing. Such continuous tone printing systems (for example, the photographic film based process and the dye sublimation thermal based process) are on the market now. These are very expensive exposure system controls which deliver 8-bit to 12-bit gray scales. Those printing devices and exposure system controls currently use two designs: a current modulating laser intensity system or a time modulating laser exposure system.
Through human visual system study, it has been discovered that the human eye can't distinguish the shade changes (i.e. it looks like a continuous shade) at medium resolutions around 400 dpi-600 dpi range with multi-bit (4 bits or 5 bits) image pixel information.
There is a need for a design of dot patterns and the rendering of images into 4 bits or 5 bits image representation, such that when the rendered images are reconstructed through the gray scale printing, the restored structure looks like a continuous tone picture.